The adult submarine fish is 10-15 cm long, with an elongated oval body shape, a thick rounded head, and a laterally flattened caudal peduncle. The body surface is smooth and scale-free. It has no ventral fins and swims only with its pectoral fins and short dorsal and anal fins, making it a fast swimmer. The body color changes shades according to mood or health condition.

Snow-white belly, golden back, irregular black spots scattered on the back, and a particularly bright patch of gold on the forehead. The submarine fish has translucent fins, and some individuals have regular pale black spots or curved black lines in the caudal fin.

When in danger, it swallows water into a special cavity in the stomach and abdomen, bulging itself into a ball and exposing its short spines so that its enemies cannot swallow it in one gulp.

It makes sounds by grinding its teeth or pharyngeal dentition or by vibrating its swim bladder.